Orientation optimization for high performance Mg3Sb2thermoelectric films via thermal evaporation.

Low-cost, highly efficient thermoelectric thin-film materials are becoming increasingly popular as miniaturization progresses. Mg3Sb2has great potential due to its low cost and high performance. However, the fabrication of Mg3Sb2thin films with high power factors (PFs) poses a certain challenge. In this work, we propose a general approach to prepare Mg3Sb2thin films with excellent thermoelectric properties. Using a two-step thermal evaporation and rapid annealing process, (001)-oriented Mg3Sb2thin films are fabricated onc-plane-oriented Al2O3substrates. The structure of the film orientation is optimized by controlling the film thickness, which modulates the thermoelectric performance. The PF of the Mg3Sb2at 500 nm (14µW·m-1·K-2) would increase to 169µW·m-1·K-2with Ag doping (Mg3Ag0.02Sb2) at room temperature. This work provides a new strategy for the development of high-performance thermoelectric thin films at room temperature.

Enhancing Thermoelectric and Cooling Performance of Bi0.5Sb1.5Te3 through Ferroelectric Polarization in Flexible Ag/PZT/PVDF/Bi0.5Sb1.5Te3 Film.

Bi2Te3-based thin films are gaining recognition for their remarkable room temperature thermoelectric performance. Beyond the conventional "process-composition-performance" paradigm, it is highly desirable to explore new methods to enhance their performance further. Here, we designed a sandwich-structured Ag/PZT/PVDF/Bi0.5Sb1.5Te3(BST) thin film device and effectively regulated the performance of the BST film by controlling the polarization state of the PZT/PVDF layers. Results indicate that polarization induces interlayer charge redistribution and charge transfer between PZT/PVDF and BST, thereby achieving the continuous modulation of the electrical transport characteristics of BST films. Finally, following polarization at a saturation voltage of 3 kV, the power factor of the BST film increased by 13% compared to the unpolarized condition, reaching 20.8 µW cm-1 K-2. Furthermore, a device with 7 pairs of P-N legs was fabricated, achieving a cooling temperature difference of 11.0 K and a net cooling temperature difference of 2.4 K at a current of 10 mA after the saturation polarization of the PZT/PVDF layer. This work reveals the critical effect of introducing ferroelectric layer polarization to achieve excellent thermoelectric performance of the BST film.

Electro-Thermo-Magnetic Effect-Induced Large Thermoelectric Performance of Calcium Cobaltite Nanocomposites.

As attractive thermoelectric oxides, Ca3Co4O9-based materials have been intensively studied for their applications in recent years. However, their thermoelectric performance is enormously limited due to the contradiction of electrical resistivity and thermal conductivity. Herein, BaFe12O19 nanospheres were introduced into the Ca3Co4O9 matrix. The metallic Ag, ferrites, and matrix phase survived together, and a high density of nanoscale BaFe12O19 precipitation was observed. The reduction of work function could lead to band bending and form an interface potential due to the electro-thermo-magnetic effect contributing to the hole migration. As a result, a huge ZT value of 0.51 for the 8 wt % BaFe12O19/Ca3Co4O9 nanocomposites was obtained at 1073 K, accompanied by a low electrical resistivity of 6.7 mΩ·cm and a high Seebeck coefficient of 217.5 µV/K. In addition, a significant reduction of thermal conductivity (1.11 W/(m·K)) occurred, which was due to the nanoscale ferromagnetic phase effectively scattering the mid- and short-wavelength heat-carrying phonons. The synergistic enhancement of thermoelectric performance confirmed that the electro-thermo-magnetic effect is an effective way to solve the challenging problem of performance deterioration in oxide thermoelectric materials.

Synergistic Improvement of BiI3 and In on Thermoelectric Properties of Zone-Melted n-Type Bi2Te2.7Se0.3.

Bismuth telluride (Bi2Te3) is the only commercial thermoelectric material so far, and it is also the best thermoelectric material with the best performance at room temperature. However, up to now, the zT value of n-type materials used on a large scale is only about 1.0; this makes the thermoelectric conversion efficiency of thermoelectric devices and thermoelectric applications stagnant. Therefore, under the synergistic action of BiI3 and In, the properties of n-type Bi2Te2.7Se0.3 material are improved. The experiments show that BiI3, which is nontoxic and non-absorbent, can effectively improve the power factor of the material and inhibit the bipolar effect and is an effective dopant. After the inclusion of In, due to the low bond energy of the In-Te bond, it is easy to form the InTe phase in the matrix material and then introduce the second phase, and the presence of the second phase in the material will scatter phonons and reduce the lattice thermal conductivity so that it can reach 0.31 W m-1 K-1 at 350 K. Ultimately, a high maximum zT of 1.20 at 325 K and a remarkable average zT of 1.04 (300-500 K) are attained in the In0.005Bi1.995Te2.7Se0.3 + 0.13 wt % BiI3 sample.

Rational Design of Cu Vacancies and Antisite Defects for Boosting the Thermoelectric Properties of CuGaTe2-Based Compounds.

CuGaTe2-based compounds show great promise in the application for high-temperature thermoelectric power generation; however, its wide bandgap feature poses a great challenge for enhancing thermoelectric performance via structural defects modulation and doping the system. Herein, it is discovered that the presence of GaCu antisite defects in the CuGaTe2 compound promotes the formation of Cu vacancies, and vice versa, which tends to form the charge-neutral structure defects combination with one GaCu antisite defect and two Cu vacancies. The accumulation of Cu vacancies in the structure of the (Cu2Te)x(Ga2Te3)1-x compounds evolves into twins and stacking faults. This in conjunction with GaCu antisite defects intensify the point defects phonon scattering, yielding a dramatic reduction on lattice thermal conductivity from 6.95 W m-1 K-1 for the pristine CuGaTe2 sample to 2.98 W m-1 K-1 for the (Cu2Te)0.45(Ga2Te3)0.55 sample at room temperature. Furthermore, the high concentration of charge-neutral defects combination narrows the band gap and increases the carrier concentration, leading to an improved power factor of 1.58 mW/mK2 at 600 K for the (Cu2Te)0.49(Ga2Te3)0.51 sample, which is 41% higher than for the pristine CuGaTe2 sample. Consequently, the highest ZT value of 0.82 is achieved at 915 K for Cu0.015(Cu2Te)0.48(Ga2Te3)0.52, which represents an enhancement of about 22% over that of the pristine CuGaTe2 compound.

Improving Thermoelectric Performance of AgSbTe2 through Suppression of Ag2Te and Band Convergence via Mg and Ti Codoping.

In this study, the impact of codoping Mg and Ti on the thermoelectric performance of AgSbTe2 materials was investigated. Through a two-step synthesis process involving slow cooling and spark plasma sintering, AgSb0.98-xMg0.02TixTe2 samples were prepared. The introduction of Mg and Ti dopants effectively suppressed the formation of the undesirable Ag2Te phase. Density functional theory (DFT) calculations confirmed that Ti doping facilitated the band convergence, leading to a reduction in the effective mass of the carriers. This optimization enhanced carrier mobility and, consequently, electrical conductivity. Additionally, the codoping strategy resulted in the reinforcement of point defects, which contributed to a decrease in lattice thermal conductivity. The AgSb0.98-xMg0.02TixTe2 sample achieved a maximum figure of merit (ZT) value of 1.45 at 523 K, representing an 87% improvement over the undoped AgSbTe2 sample. The average ZT value over the temperature range of 323-573 K was 1.09, marking a significant enhancement in thermoelectric performance. This research demonstrates the potential of Mg and Ti codoping as a strategy to improve the thermoelectric properties of AgSbTe2-based materials.

Four-Phonon Enhanced the Thermoelectric Properties of ScSX (X = Cl, Br, and I) Monolayers.

Recently, the FeOCl-type two-dimensional materials have attracted significant attention owing to their versatile applications in fields such as thermoelectricity and photocatalysis. This study aims to systematically investigate the thermoelectric properties of ScSX (X = Cl, Br, and I) monolayers by a combination of the first-principles calculations and the machine-learning interatomic potential approach. These monolayers are indirect semiconductors with band gaps of 3.22 (ScSCl), 3.27 (ScSBr), and 2.87 eV (ScSI), respectively. The lattice thermal conductivity is decreased by 25.72% (20.90%), 44.05% (40.00%), and 30.96% (34.76%) for ScSCl, ScSBr, and ScSI along the x-axis (y-axis) when the four-phonon scattering is introduced, indicating its important role in phonon transport. Anharmonic phonon scattering yields high Grüneisen parameter and scattering rate values, hence causing these low lattice thermal conductivities. Additionally, the large Seebeck coefficients and electrical conductivities of n-type doped ScSX monolayers contribute to their excellent power factors (24.69, 25.66, and 24.99 mW/K2·m for ScSCl, ScSBr and ScSI at 300 K, respectively). Based on the excellent power factor and low thermal conductivity, the maximum values of the figure of merit are calculated to be 2.68, 3.39, and 3.21 for ScSCl, ScSBr, and ScSI monolayers at 700 K, respectively. Our research provides valuable insights into the phonon thermal transport of ScSX monolayers and suggests a promising approach to address high-order anharmonicity.

Dopant-induced Morphology of Organic Semiconductors Resulting in High Doping Performance.

Doping plays a crucial role in modulating and enhancing the performance of organic semiconductor (OSC) devices. In this study, the critical role of dopants is underscored in shaping the morphology and structure of OSC films, which in turn profoundly influences their properties. Two dopants, trityl tetrakis(pentafluorophenyl) (TrTPFB) and N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DMA-TPFB), are examined for their doping effects on poly(3-hexylthiophene) (P3HT) and PBBT-2T host OSCs. It is found that although TrTPFB exhibits higher doping efficiency, OSCs doped with DMA-TPFB achieve comparable or even enhanced electrical conductivity. Indeed, the electrical conductivity of DMA-TPFB-doped P3HT reaches over 67 S cm-1, which is a record-high value for mixed-solution-doped P3HT. This can be attributed to DMA-TPFB inducing a higher degree of crystallinity and reduced structural disorder. Moreover, the beneficial impact of DMA-TPFB on the OSC films' morphology and structure results in superior thermoelectric performance in the doped OSCs. These findings highlight the significance of dopant-induced morphological and structural considerations in enhancing the film characteristics of OSCs, opening up a new avenue for optimization of dopant performance.

Tailoring thermoelectric performance ofn-type Bi2Te3through defect engineering and conduction band convergence.

Bi2Te3, an archetypical tetradymite, is recognised as a thermoelectric (TE) material of potential application around room temperature. However, large energy gap (ΔEc) between the light and heavy conduction bands results in inferior TE performance in pristine bulkn-type Bi2Te3. Herein, we propose enhancement in TE performance of pristinen-type Bi2Te3through purposefully manipulating defect profile and conduction band convergence mechanism. Twon-type Bi2Te3samples, S1 and S2, are prepared by melting method under different synthesis condition. The structural as well as microstructural evidence of the samples are obtained through powder x-ray diffraction and transmission electron microscopic study. Optothermal Raman spectroscopy is utilized for comprehensive study of temperature dependent phonon vibrational modes and total thermal conductivity (κ) of the samples which further validates the experimentally measured thermal conductivity. The Seebeck coefficient value is significantly increased from 235 µVK-1(sample S1) to 310 µVK-1(sample S2). This is further justified by conduction band convergence, where ΔEcis reduced from 0.10 eV to 0.05 eV, respectively. To verify the band convergence, the double band Pisarenko model is employed. Large power factor (PF) of 2190 µWm-1K-2and lowerκvalue leading toZTof 0.56 at 300 K is gained in S2. The obtainedPFandZTvalue are among the highest values reported for pristinen-type bulk Bi2Te3. In addition, appreciable value of TE quality factor and compatibility factor (2.7 V-1) at room temperature are also achieved, indicating the usefulness of the material in TE module.

Tuning of thermoelectric performance by modulating vibrational properties in Ni-doped Sb2Te3.

Sb2Te3, a binary chalcogenide-based 3D topological insulator, attracts significant attention for its exceptional thermoelectric performance. We report the vibrational properties of magnetically doped Sb2Te3thermoelectric material. Ni doping induces defect/disorder in the system and plays a positive role in engineering the thermoelectric properties through tuning the vibrational phonon modes. Synchrotron powder x-ray diffraction study confirms good crystalline quality and single-phase nature of the synthesized samples. The change in structural parameters, includingBisoand strain, further corroborate with structural disorder. Detailed modification of phonon modes with doping and temperature variation is analysed from temperature-dependent Raman spectroscopic measurement. Compressive lattice strain is observed from the blue shift of Raman peaks owing to Ni incorporation in Sb site. An attempt is made to extract the lattice thermal conductivity from total thermal conductivity estimated through optothermal Raman studies. Hall concentration data support the change in temperature-dependent resistivity and thermopower. Remarkable increase in thermopower is observed after Ni doping. Simulation of the Pisarenko model, indicating the convergence of the valence band, explains the observed enhancement of thermopower in Sb2-xNixTe3. The energy gap between the light and heavy valence band at Γ point is found to be 30 meV (for Sb2Te3), which is reduced to 3 meV (in Sb1.98Ni0.02Te3). A significant increase in thermoelectric power factor is obtained from 715 µWm-1K-2for pristine Sb2Te3to 2415 µWm-1K-2for Ni-doped Sb2Te3sample. Finally, the thermoelectric figure of merit,ZTis found to increase by four times in Sb1.98Ni0.02Te3than that of its pristine counterpart.

Study on the Effect of Sn, In, and Se Co-Doping on the Thermoelectric Properties of GeTe.

GeTe and Ge0.99-xIn0.01SnxTe0.94Se0.06 (x = 0, 0.01, 0.03, and 0.06) samples were prepared by vacuum synthesis combined with spark plasma sintering (SPS). The thermoelectric properties of GeTe were coordinated by multiple doping of Sn, In, and Se. In this work, a maximum zT(zT = S2σT/κ) of 0.9 and a power factor (PF = S2σ) of 3.87 µWmm-1 K-2 were obtained in a sample of Ge0.99In0.01Te0.94Se0.06 at 723K. The XRD results at room temperature show that all samples are rhombohedral phase structures. There is a peak (~27°) of the Ge element in GeTe and the sample (x = 0), but it disappears after Sn doping, indicating that Sn doping can promote the dissolution of Ge. The scattering mechanism of the doped samples was calculated by the conductivity ratio method. The results show that phonon scattering Is dominant in all samples, and alloy scattering is enhanced with the increase in the Sn doping amount. In doping can introduce resonance energy levels and increase the Seebeck coefficient, and Se doping can introduce point defects to suppress phonon transmission and reduce lattice thermal conductivity. Therefore, the thermoelectric properties of samples with x = 0 improved. Although Sn doping will promote the dissolution of Ge precipitation, the phase transition of the samples near 580 K deteriorates the thermoelectric properties. The thermoelectric properties of Sn-doped samples improved only at room temperature to ~580 K compared with pure GeTe. The synergistic effect of multi-element doping is a comprehensive reflection of the interaction between elements rather than the sum of all the effects of single-element doping.

High Thermoelectric Performance in Bismuth Telluride via Constructing MoSe2-2D Heterojunction.

Currently, the only thermoelectric (TE) materials commercially available at room temperature are those based on bismuth telluride. However, their widespread application is limited due to their inferior thermoelectric and mechanical properties. In this study, a strategy of growing a rigid second phase of MoSe2 is employed, in situ within the matrix phase to achieve n-type bismuth telluride-based materials with exceptional mechanical and thermoelectric properties. The in situ grown second phase contributes to both the electronic and lattice thermal conductivities. This is primarily attributed to the strong energy filtering effect, as the second phase forms a semi-common lattice interfacial structure with the matrix phase during growth. Furthermore, for composites containing 2 wt% MoSe2, a maximum zT value of 1.24 at 373 K can be achieved. On this basis, 8-pair TE module is fabricated and 1-pair TE module is optimized using a homemade p-type material. The optimized 1-pair TE module generates a maximum output power of 13.6 µW, which is twice that of the 8-pair TE module and four times more than the 8-pair TE module fabricated by commercial material. This work facilitates the development of the TE module by presenting a novel approach to obtaining bismuth telluride-based thermoelectric materials with superior thermoelectric and mechanical properties.

High Thermoelectric Performance of n-type BiTeSe-Based Composites Incorporated with Both Inorganic and Organic Nanoinclusions.

N-type Bi2Te2.7Se0.3 (BTS) alloy has relatively low thermoelectric performance as compared to its p-type counterpart, which restricts its widespread applications. Herein, we designed and prepared a novel composite system, which consists of an n-type BTS matrix incorporated with both inorganic and organic nanoinclusions. The results indicate that the thermopower of the composite samples can be enhanced by more than 19% upon incorporating inorganic nanophase AgBi3S5 (ABS) due to the energy-dependent carrier scattering, which ensures a high power factor. On the other hand, further incorporation of organic nanophase polypyrrole (PPy) can drastically reduce its lattice thermal conductivity owing to the strong scattering of mid- and low-frequency phonons at these nanoinclusions. As a result, high figures of merit ZTmax = 1.3 at 348 K and ZTave = 1.17 (300-500 K) are achieved with improved mechanical properties in BTS-based composites incorporated with 1.5 wt % ABS and 0.5 wt % PPy, demonstrating that the incorporation of both inorganic and organic nanoinclusions is an effective way to improve its thermoelectric performance.

Improvement of Mechanical and Thermoelectric Properties of AgCuTe by Pinning Effect and Enhanced Liquid-Like Behavior via SiC Alloying.

With the development and application of thermoelectric (TE) devices, it requires not only high-performance of TE materials but also high mechanical properties. Here, we report a medium-temperature liquid material, AgCuTe, with high mechanical properties. The results demonstrate that AgCuTe possesses a multiphase structure characterized by abundant grain boundaries, resulting in reduced lattice thermal conductivity and inherently high mechanical strength. Furthermore, nano-SiC was alloyed into the AgCuTe material to further improve its mechanical and TE properties. Nano-SiC exhibited a button-like distribution within the grain boundaries, introducing a pinning effect that significantly elevated the Vickers hardness of the samples. Additionally, nano-SiC induced strong lattice distortion energy in the vicinity, which promotes Ag/Cu ions to escape from the lattice and enhances the liquid-like behavior of Ag/Cu ions. Finally, these enhancements led to a 21% improvement in the mechanical properties and a 40% improvement in the TE properties for AgCuTe. Notably, AgCuTe achieved its peak TE performance, with a latest peak ZT value of 1.32 at 723 K. This research expands the potential applications of AgCuTe.

Boosting Thermoelectric Performance of PbBi2Te4 via Reduced Carrier Scattering and Intensified Phonon Scattering.

Materials with low intrinsic lattice thermal conductivity are crucial in the pursuit of high-performance thermoelectric (TE) materials. Here, the TE properties of PbBi2Te4-xSex (0 ≤ x ≤ 0.6) samples are systematically investigated for the first time. Doping with Se in PbBi2Te4 can simultaneously reduce carrier concentration and increase carrier mobility. The Seebeck coefficient is significantly increased by doping with Se, based on the density functional theory calculation, it is shown to be due to the increased bandgap and electronic density of states. In addition, the lattice strain is enhanced due to the difference in the size of Se and Te atoms, and the multidimensional defects formed by Se doping, such as vacancies, dislocations, and grain boundaries, enhance the phonon scattering and reduce the lattice thermal conductivity by about 37%. Finally, by using Se doping to reduce carrier concentration and thermal conductivity, a large ZTmax = 0.56 (at 574K) is achieved for PbBi2Te3.5Se0.5, which is around 64% larger than those of the PbBi2Te4 pristine sample. This work not only demonstrates that PbBi2Te4 is a potential medium temperature thermoelectric material, but also provides a reference for enhancing thermoelectric properties through defect and energy band engineering.

Excellent Room Temperature Thermoelectric Performance in Mg3Sb2-Based Alloys via Multi-Functional Doping of Nb.

Mg3Sb2-based alloys are attracting increasing attention due to the excellent room temperature thermoelectric properties. However, due to the presence and easy segregation of charged Mg vacancies, the carrier mobility in Mg3Sb2-based alloys is always severely compromised that significantly restricts the room temperature performance. General vacancy compensation strategies cannot synergistically optimize the complicated Mg3Sb2 structures involving both interior and boundary scattering. Herein, due to the multi-functional doping effect of Nb, the electron scattering inside and across grains is significantly suppressed by inhibiting the accumulation of Mg vacancies, and leading to a smooth transmission channel of electrons. The increased Mg vacancies migration barrier and optimized interface potential are also confirmed theoretically and experimentally, respectively. As a result, a leading room temperature zT of 1.02 is achieved. This work reveals the multi-functional doping effect as an efficient approach in improving room temperature thermoelectric performance in complicated defect/interface associated Mg3Sb2-based alloys.

Strong Intervalley Scattering-Induced Renormalization of Electronic and Thermal Transport Properties and Selection Rule Analysis in 2D Tellurium.

The electron-phonon interaction (EPI) and phonon-phonon interactions are ubiquitous in promising two-dimensional (2D) semiconductors, determining both electronic and thermal transport properties. In this work, based on ab initio calculations, the effects of intervalley scattering on EPI and higher-order four-phonon interactions of α-Te and ß-Te are investigated. Through the proposed selection rules for scattering channels and calculations of full electron-phonon scattering rates, we demonstrate that multiple nearly degenerate local valleys/peaks produce more scattering channels, resulting in stronger intervalley scattering over intravalley scattering. The lattice thermal conductivities of α-Te and ß-Te are decreased by as much as 10.9% and 30.8% by considering EPI under the carrier concentration of 2 × 1013 cm-2 (n-type) at 300 K compared to those limited by three-phonon scattering, respectively. However, when further considering four-phonon scattering, EPI reduces the lattice thermal conductivities by 2.6% and 19.4% for α-Te and ß-Te, respectively. Furthermore, it is revealed that the four-phonon interaction is more dominant in phonon transport for α-Te than that for ß-Te due to the presence of an acoustic-optical phonon gap in α-Te. Finally, we demonstrate strong intervalley scattering induces significant renormalization effects from EPI on all the constituent parameters of thermoelectric performance. Our results show the contributions of intervalley scattering to the electronic properties as well as thermal transport properties in band-convergent thermoelectric materials are essential and highlight the potential of monolayer tellurium as a promising candidate for advanced thermoelectric applications.